Ink jet printer

ABSTRACT

An ink jet printer including: a sheet support plate having suction holes formed in a sheet support surface thereof; a sheet advance mechanism adapted to advance a sheet step-wise in a sheet advance direction over the sheet support surface; and an imaging system adapted to form an image by depositing liquid ink on the sheet, the ink being allowed to dry-out while the sheet is advanced over the sheet support plate, wherein the suction holes are arranged such that their influence on the ink drying process is essentially uniform over the whole area of the image.

BACKGROUND OF THE INVENTION

The present invention relates to an ink jet printer including:

-   -   a sheet support plate having suction holes formed in a sheet        support surface thereof;    -   a sheet advance mechanism adapted to advance a sheet step-wise        in a sheet advance direction over the sheet support surface; and    -   an imaging system adapted to form an image by depositing liquid        ink on the sheet, the ink being allowed to dry-out while the        sheet is advanced over the sheet support plate.

In an ink jet printer, a sheet support plate containing suction holes isfrequently used for supporting an image receiving sheet and holding thesheet flat on the sheet support plate. By applying a subatmosphericpressure via the suction holes to the bottom side of the sheet, thesheet may be drawn against the top surface of the sheet support plate.The suction holes should be evenly distributed over the surface area ofthe sheet support plate, so that an essentially uniform suction isapplied to the sheet. On the other hand, in view of manufacturingconsiderations, the number of suction holes should not be too large.

EP-A 0 409596 discloses a printer wherein the suction holes are arrangedin a regular, non-slanting pattern, with the suction holes ofneighboring rows being off-set from one another by one-half pitch.

SUMMARY OF THE INVENTION

It is an object of the present invention to improve the image quality ofimages printed with an ink jet printer of the type indicated above.

According to the present invention, the suction holes are arranged suchthat their influence on the ink drying process is essentially uniformover the whole area of the image.

The present invention is based on the observation that the suction holesmay retard or accelerate the ink drying process, and that this may havea visible effect on the printed image.

For example, in a hot-melt ink jet printer, a sheet, e. g. a sheet ofpaper, is advanced over the sheet support plate while the image is beingprinted. At room temperature, the hot-melt ink is solid, and it istherefore necessary that the ink is heated in the printer above itsmelting point, before it can be jetted onto the paper. In order toobtain a suitable and constant amount of spreading of the ink, thetemperature of the sheet support plate and hence the temperature of thepaper is controlled such that the ink cools down at an appropriate rate.However, at the positions of the suction holes, the properties of thesheet support plate, especially its heat conduction and its heatcapacity, differ from the surrounding parts of the plate. Thereby, heatdissipation is reduced at sheet portions that are positioned above thesuction holes. This has negative effects on the scattering of light, thescratch resistance and the glossiness of the printed image. Especiallythe difference in glossiness is visible in the printed image.

Similarly, in an ink jet printer in which the ink drying processinvolves evaporation of a solvent, the flow of air drawn-in through thesuction holes may give rise to an accelerated evaporation of thesolvent.

If the suction holes are arranged in a regular pattern of rows andcolumns, with the columns extending in the sheet advance direction, thencertain areas of the printed image will repeatedly pass over the suctionholes of a column, whereas the intervening portions of the image willnever be affected by the suction holes. As a result, differences inglossiness and the like are visible as a regular pattern on the printedimage. For this reason, the invention proposes to arrange the suctionholes such that essentially all portions of the image will pass over thesuction holes an approximately equal number of times and will thus beaffected by the suction holes in essentially a like manner.

A suitable arrangement would, for example, be a random distribution ofthe suction holes over the area of the sheet support plate. However, interms of production efficiency, a regular pattern of suction holes ispreferred. Thus, the object of the present invention can be achieved bya slanting pattern in which the columns of suction holes form an anglewith respect to the sheet advance direction. As a result, an imageportion that has passed over one suction hole will not pass over thenext or the next few suction holes of the same column, because thesesuction holes are laterally offset. After a certain number of advancesteps, of course, the image portion may pass over a suction hole of aneighboring column. However, the pattern may be arranged such that,until then, the ink has already dried-out to a sufficient extent andwill no longer be affected by the presence of the suction hole.

The pattern of suction holes may be adapted to the length of the sheetadvance steps such that for any two suction holes that are aligned inthe sheet advance direction, the mutual distance of the suction holes inthe sheet advance direction is a non-integral multiple of the steplength. As a result, when an image portion has rested over a suctionhole for a certain time in the interval between two sheet advance steps,this image area will not come to rest over the next suction hole afterthe next advance step, but will be offset from that hole in the sheetadvance direction. Thus, as long as the ink has not dried sufficiently,any point of the printed image will either rest over a suction hole onlyonce or will never rest over a suction hole at all, but no point of theimage will rest over a suction hole several times, and this may assure asufficient uniformity of the ink drying process.

The suction holes may be arranged in rows, preferably in equidistantrows, that are, for example, perpendicular to the sheet transportdirection. This is particularly useful in a hot-melt ink jet printer inwhich the sheet support plate is temperature-controlled by means of afluid circulated therethrough.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred embodiments of the present invention will now be described inconjunction with the drawings in which:

FIG. 1 is a schematic perspective view of a hot-melt ink jet printer;

FIG. 2 is a partial cross section of a sheet support plate in theprinter shown in FIG. 1; and

FIGS. 3 to 5 are a partial top views of a sheet that is advanced oversuction holes of the sheet support plate.

DETAILED DESCRIPTION OF THE INVENTION

As is shown in FIG. 1, a hot-melt ink jet printer comprises a platen 10which is intermittently driven to rotate in order to advance a sheet 12,e. g. a sheet of paper, in a direction indicated by an arrow A over thetop surface of a sheet support plate 14. A number of transport rollers16 are rotatably supported in a cover plate 18 and form a transport nipwith the platen 10, so that the sheet 12, which is supplied from a reel(not shown) via a guide plate 20, is paid out through a gap formedbetween an edge of the cover plate 18 and the surface of the sheetsupport plate 14.

A carriage 22 which includes a number of ink jet printheads (not shown)is mounted above the sheet support plate 14 so as to reciprocate in thedirection of arrows B across the sheet 12. In each pass of the carriage22, a number of pixel lines are printed on the sheet 12 by means of theprintheads which eject droplets of hot melt ink onto the sheet inaccordance with image information supplied to the printheads. For thesake of simplicity, guide and drive means for the carriage 22, inksupply lines and data supply lines for the printheads, and the like,have not been shown in the drawing.

The top surface of the sheet support plate 14 has a regular pattern ofsuction holes 24 which pass through the plate and open into a suctionchamber 26 that is formed in the lower part of the plate 14. The suctionchamber is connected to a blower 28 which creates a subatmosphericpressure in the suction chamber, so that air is drawn-in through thesuction holes 24. As a result, the sheet 12 is drawn against the flatsurface of the support plate 14 and is thereby held in a flat condition,especially in the area which is scanned by the carriage 22, so that auniform distance between the nozzles of the printheads and the surfaceof the sheet 12 is established over the whole width of the sheet, and ahigh print quality can be achieved.

The droplets of molten ink that are jetted out from the nozzles of theprintheads have a temperature of 100° C. or more and cool down andsolidify after they are deposited on the sheet 12. Thus, while the imageis being printed, the heat of the ink must be dissipated with asufficient rate that should be essentially uniform for the whole area ofthe sheet 12. To dissipate the heat, the temperature of the sheet 12 iscontrolled via the sheet support plate 14 by means of a temperaturecontrol system 30 which circulates a temperature control fluid,preferably a liquid, through the plate 14. The temperature controlsystem includes a circulating system with tubes 32 that are connected toopposite ends of the plate 14. One of the tubes passes through anexpansion vessel 33 containing a gas buffer for absorbingtemperature-dependent changes in the volume of the liquid. As will bereadily understood, the temperature control system 30 includes heaters,temperature sensors, heat sinks, and the like for controlling thetemperature of the fluid, as well as a pump or other displacement meansfor circulating the fluid through the interior of the sheet supportplate 14.

The sheet support plate 14, which has been shown in cross-section inFIG. 2, is made of a material, such as a metal, having a relatively highheat conductivity and also a relatively high heat capacity. A number ofelongated cavities 34 are formed in the interior of the plate 14 so asto extend in parallel with one another and in parallel with thedirection (B) of travel of the carriage 22 between opposite ends of theplate 14, where they are connected to the tubes 32 through suitablemanifolds. Each cavity 34 is delimited by a top wall 36, a bottom wall38 and two separating walls 40. The top walls 36, together, define thetop surface 42 of the plate 14 which is machined to be perfectly flat.Between each pair of two separating walls 40, which delimit to adjacentcavities 34, a hollow space 44 is formed, through which the suctionholes 24 pass through into the suction chamber 26. Since the suctionholes 24 are arranged in a slightly slanting pattern, as is shown inFIGS. 1 and 3, only one of the suction holes 24 can be seen in sectionin FIG. 2.

FIG. 3 shows the pattern in which the suction holes 24 are arranged inthe surface of the sheet support plate 14. These suction holes form aregular pattern with rows R and columns C. The rows R extend in parallelwith the direction B and hence also in parallel with the cavities 34formed in the interior of the sheet support plate 14 (FIG. 2). However,the columns C are inclined relative to the sheet advance direction A.

In the example shown, the pattern of suction holes 24 repeats itselfevery five rows, so that, for example, the suction hole 24-1 in thefirst row is aligned in the sheet advance direction A with the suctionhole 25-5 in the fifth row.

Also shown in FIG. 3 is the leading edge of the sheet 12 which has beenadvanced in the direction A and is now held stationary on the sheetsupport plate while the carriage 22 (FIG. 1) travels across the sheet inorder to print another swath of image pixels. The hot-melt ink that hasbeen deposited on the sheet 12 is cooled through contact with the sheetsupport plate 14 with an appropriate cooling rate. However, in the areaof the suction holes 24 of the first row, the cooling rate is reduced,because, there, the sheet is not in contact with the metal plate 14which has a high heat conductivity.

When the carriage 22 has completed its travel, the sheet 12 is advancedagain by one step to the position shown in FIG. 4 and is then again heldstationary for printing the next swath of the image. The length S of thesheet advance step has been indicated in FIG. 4. Dark spots 24 a in FIG.4 symbolize those areas of the sheet 12 which had covered the suctionholes 24 of the first row in the condition shown in FIG. 3 and forwhich, consequently, the ink has not been cooled to the same extent asfor the rest of the sheet. Since the cooling rate of the ink will alsobe somewhat decreased at the edges of the suction holes 24, the diameterof the spots 24 a may, in practice, be slightly larger than the diameterof the suction holes 24.

Thanks to the inclination of the columns C, the spots 24 a in FIG. 4 arelaterally offset from the suction holes 24 of the second row and do notoverlap with these suction holes in the sheet advance direction A.

When the sheet 12 rests in the position shown in FIG. 4, similar spots,for which the cooling rate is decreased, will also be formed on andaround the suction holes 24 of the second row. However, since thecooling rate decays exponentially with time, the spots caused by thesuction holes of the second row will be somewhat less pronounced.

In the further course of the print process, the sheet 12 is advancedstep-wise with the step width S, and in each step, the suction holes ofthe first row will cause another row of spots 24 a, and the suctionholes in the second and subsequent rows will cause somewhat fainterspots.

FIG. 5 shows the condition reached when the sheet 12 has been advancedby 5 steps and indicates the spots 24 a caused by the suction holes ofthe first row and also the spots 24 b caused by the suction holes in thesecond, third and fourth row. It can be seen in the lower part of FIG. 5that these spots 24 a, 24 b are non-overlapping and are practicallyuniformly distributed over the surface of the sheet 12, so that theinfluence of the suction holes on the cooling rate of the ink ispractically uniform over the area of the sheet 12 and will hardlyproduce any visible effect.

As is further shown on the left margin of the sheet 12 in FIG. 5, thesuction hole 24-5 in the fifth row does not overlap with the spot 24 a-1that has been created by the suction hole 24-1 in the state shown inFIG. 3. The spot 24 a-1 is offset from the suction hole 24-5 in sheetadvance direction A. This is due to the fact that the distance D betweenthe suction holes 24-1 and 24-5 in FIG. 3 is a non-integral multiple ofthe step width S. Thus, although the suction holes 24-1 and 24-5 arealigned in sheet advance direction A, the spot 24 a-1 does not come torest on the suction hole 24-5, so that the cooling process for the inkin this spot will not be retarded once again by the suction hole 24-5.

In a practical embodiment of the printer, the step width S may bevariable and will change when the printer is switched into another printmode, e. g. from a single-pass mode into a two-pass mode. However, sincethe number of possible step widths S is limited, and the step widths areknown in advance, it is possible to select the distance D such that thecondition that D is a non-integral multiple of S is fulfilled for allpossible step widths.

In a modified embodiment, especially in an embodiment in which thecolumns C are not inclined relative to the sheet advance direction A,the distance D (which would then be the distance between to adjacentrows R) may be smaller than the step width S. In this case, theadditional condition that S is a non-integral multiple of D should befulfilled in order to avoid overlapping spots. For example, if the stepwidth S is 17 †mm, and the diameter of the suction holes 24 is 1.0 mm,then the distance D may be selected as 12 mm. Then, it would only beafter 12 sheet advance steps that the spot created by the first suctionhole would overlap with another suction hole for the first time. Duringthese 12 steps, the ink has had time enough to cool down, so that itwould no longer be affected by the second suction hole.

The invention being thus described, it will be obvious that the same maybe varied in many ways. Such variations are not to be regarded as adeparture from the spirit and scope of the invention, and all suchmodifications as would be obvious to one skilled in the art are intendedto be included within the scope of the following claims.

1. An ink jet printer which comprises: a sheet support plate havingsuction holes formed in a sheet support surface thereof; a sheet advancemechanism adapted to advance a sheet step-wise in a sheet advancedirection over the sheet support surface; and an imaging system adaptedto form an image by depositing liquid ink on the sheet, the ink beingallowed to dry-out while the sheet is advanced over the sheet supportplate, the suction holes being arranged such that their influence on theink drying process is essentially uniform over the whole area of theimage, wherein the suction holes are arranged in a regular slantingpattern of rows and columns, with the columns being inclined relative tothe sheet advance direction such that an image portion that has passedover one suction hole will not pass over the next few suction holes ofthe same column nor over a suction hole of a neighboring column beforethe ink has dried out to such an extent that it will no longer beaffected by the presence of the suction hole.
 2. The ink jet printer ofclaim 1, wherein the smallest distance between two suction holes thatare aligned in sheet advance direction is a non-integral multiple of astep width with which the sheet is advanced.
 3. The ink jet printer ofclaim 1, wherein the suction holes are arranged in rows that areperpendicular to the sheet advance direction.
 4. The ink jet printer ofclaim 1, wherein a temperature control system is provided forcontrolling the temperature of the sheet support plate.
 5. The ink jetprinter of claim 4, wherein the sheet support plate has internalcavities extending in the plane of said plate in a directionperpendicular to the sheet advance direction, and the temperaturecontrol system is adapted to circulate a temperature controlled fluidthrough said cavities.
 6. The ink jet printer of claim 1, wherein saidprinter is a hot-melt ink jet printer.
 7. A sheet support plate for anink jet printer which comprises: a sheet support plate having suctionholes formed in a sheet support surface thereof; a sheet advancemechanism adapted to advance a sheet step-wise in a sheet advancedirection over the sheet support surface; and an imaging system adaptedto form an image by depositing liquid ink on the sheet, the ink beingallowed to dry-out while the sheet is advanced over the sheet supportplate, wherein the suction are arranged such that their influence on theink drying process is essentially uniform over the whole area of theimage.
 8. The sheet support plate of claim 7, wherein the suction holesare arranged in a regular pattern of rows and columns, and the columnsare inclined relative to the sheet advance direction.
 9. The sheetsupport plate of claim 7, wherein the smallest distance between twosuction holes that are aligned in sheet advance direction is anon-integral multiple of a step width with which the sheet is advanced.10. The sheet support plate of claim 7, wherein the suction holes arearranged in rows that are perpendicular to the sheet advance direction.11. The sheet support plate of claim 7, wherein a temperature controlsystem is provided for controlling the temperature of the sheet supportplate.
 12. The sheet support plate of claim 11, wherein wherein thesheet support plate has internal cavities extending in the plane of saidplate in a direction perpendicular to the sheet advance direction, andthe temperature control system is adapted to circulate a temperaturecontrolled fluid through said cavities.